The present invention relates to planetary gear or epicyclic transmissions, more particularly such transmissions having helical gears and means to distribute the load amongst a large number of planet wheels.
Epicyclic or planetary gear transmissions are, of course, well known in the art and allow the transmission of high power while exhibiting compactness and low weight. These characteristics make the transmissions particularly suitable for aeronautical applications. Such transmissions are capable of multiplying or reducing the rotational input speed.
In its simplest form, called a single stage, a transmission of this type comprises a central gear, called a sun gear, an outer, gear called an annulus gear, and one or more intermediary gears, or planet wheels, which are in simultaneous engagement with the sun gear and the annulus gear. The planet wheels rotate on shafts connected to a planet wheel carrier.
Depending upon the desired operation, one of the three components, the sun gear, the annulus gear, or the planet wheel carrier, are braked or held stationary while the other two components rotate. One of the rotating components receives the input rotation, whereas the other delivers the output rotational motion.
For a given amount of power to be transmitted, increasing the number of planet wheels and using high strength metals to fabricate the transmission elements reduces gear bulk and, consequently, the bulk and weight of the epicyclic transmission. On the other hand, the power dissipated in the transmission is less easily drained from it and the operational temperature of such a transmission will be higher.
Under such conditions of compactness and high operating temperatures, the transmitted load must be evenly distributed over all of the planet wheels, which may be achieved by mounting the planet wheels on flexible metal shafts. The flexible metal shafts are attached at one end to the planet wheel carrier, while the other end is free and supports the planet wheel by means of a journal and swivel device. The journal and swivel device may include a ball-type bearing with a swivel, to ensure these two function simultaneously and may include a spherical surface as one of the rolling paths. Devices of this type may also include a non-swiveling ball or roller bearing which cooperates with a separate swivel.
Such transmissions including the above described journal and swivel device are adequate when the transmission makes use of spur or herringbone gears, because each planet wheel automatically is aligned with its journal axis relative to the sun gear and to the annulus gear. However, spur gears entail the drawback of generating vibrations in the transmitted rotational motion, whereas herringbone, or double helical gears are costly and require high manufacturing accuracy in order that the transmitted load will be distributed over the teeth of the gear.
Helical gears are free from the above-mentioned drawbacks. Such gears are economical relative to the herringbone gears and allow vibration-free load transmission with excellent load distribution along the teeth. However, helical gears are subject to parasitic axial forces applied to the teeth and generate a torque tending to reverse the gears in the plane defined by the axes of the gears. In the case of planet wheels mounted in a swiveling manner, these helical planet wheels would be kept in place only by the pressure of their gear teeth against those of the sun gear and of the annulus gear, which would cause premature wear of the gear teeth.